1. Field
The disclosed concept pertains generally to systems employing semiconductor devices and, more particularly, to systems, such as, for example, power systems employing semiconductor devices and an electronic circuit operatively associated with the semiconductor devices to control or monitor such devices. The disclosed concept also pertains to methods of powering a system.
2. Background Information
Induction motor drives, also called alternating current (AC) drives, are used to control the speed and torque of multi-phase induction motors, which for a long time have been the workhorse of the industry.
AC drives can be divided into two categories: low-voltage and medium-voltage. The low-voltage AC drives are widely used and cover the 0 VAC to about 600 VAC range. Low-voltage AC drives are manufactured by almost five hundred companies around the world. Medium-voltage AC drives cover input line voltages above about 660 VAC and up to about 15,000 VAC. Only about a half-dozen known companies design and produce medium-voltage AC drives. High-voltage AC drives cover voltages of about 15,000 VAC and higher, but are very uncommon compared to low-voltage and medium-voltage AC drives. Recently, the auto industry and some other special applications providing low output voltage harmonics are considering the use of multi-level inverter bridges for low-voltage motors.
Until recently, power semiconductor switches were rated at a maximum of 1,700 V, which has allowed the low-voltage three-phase AC drives to use a six-switch inverter bridge. Today, state-of-the-art semiconductor switches are rated at 2,500 V, 3,300 V, 4,500 V, 6,500 V and can be used in a two-level, six-switch inverter bridge having up to a 2,000 VAC input. Above 2,000 VAC, the inverter bridge employs a greater number of power semiconductor switches connected in series. The most popular inverter topology for three-phase, medium-voltage induction motors of up to 4,000 V is a three-level, twelve-switch inverter bridge.
The number of levels in an inverter bridge defines the number of direct current (DC) voltage steps that are employed by the inverter bridge in order to achieve a certain voltage level in its output. Because power semiconductor switches have limited voltage capability, the total DC bus voltage of an inverter bridge is divided into a number of voltage steps, such that each voltage step can be handled by one power switch.
In a conventional two-level AC drive, three-phase AC power, after passing through an optional input line reactor, is rectified by a rectifier and capacitor to form a two-level DC bus. Depending on the design approach, input harmonics on the DC bus may be further reduced by a DC reactor. The two-level DC bus voltage is applied across a six-switch inverter bridge which produces a two-level PWM voltage output. The six switches are divided into three branches with two switches each. A controller controls each switch via the control terminals of each switch. A three-phase motor has a phase connection derived from the middle point between the two switches of a branch, and the three branches produce three phases which collectively drive the motor. The two levels of the DC bus constitute a positive bus and a negative bus. The top switch of each branch is connected to the positive bus and the bottom switch of each branch is tied to the negative bus. The two switches in a branch are in series and therefore cannot be turned-on at the same time without causing a short-circuit. In order to prevent a short-circuit, switch delay times are taken into consideration by the controller. The top switch needs to turn-off before the bottom one turns-on, and vice-versa. Each of the switches has to be able to handle the full voltage between the positive and negative busses.
In comparison to the two-level drive, in a three-level AC drive, the DC bus has three voltage levels (relatively labeled positive, neutral and negative), and the inverter bridge has twelve switches. The switches are divided into three equal branches, each branch connecting to one phase of the three-phase motor. Thus, each branch has four switches in series, and each connection to the motor is derived from a middle point.
In multi-level inverters, a power supply is employed to power the logic to control each semiconductor device. As the number of levels increases, more power supplies are needed to power the semiconductor devices. However, as the semiconductor devices operate, they create energy losses in the form of heat. These losses can be, for example and without limitation, conduction losses, switching losses or internal losses causes by internal resistance of the semiconductor devices.
There is room for improvement in systems employing an electronic circuit operatively associated with semiconductor devices.
There is also room for improvement in methods of powering a system including a plurality of semiconductor devices and an electronic circuit operatively associated with the semiconductor devices.